Gas burner



C. S. FLYNN Aug. 1o, 1965 GAS BURNER Filed Jan. 17, 1963 Z M u M, m w m `a high pressure burner, hash-back occurs.

United States Patent 3,1%',573 GAS BURNER Charles S. Fiynn, 299i Sherwood Court, Musiregon, Mich. Fiied Een. l?, 1963, Ser. No. 252,115 3 Claims. (Cl. 15g-E16) This application is a continuation-in-part application of my application entitled Combustion Burner, led Jan. 17, 1962, Serial No. 166,796, now abadoned.

This invention relates to pressure combustion burners, and more particularly to a pressure burner providing a surface area of any desirable size and configuration with a uniform, continuous high temperature heat.

One serious limitation of conventional combustion or gas burners is the lack of ready adaptability to special use. The limitations involved with casting a porous ceramic block in unusual configurations and sizes are many. Other constructions than the fused ceramic block have not really gained commercial acceptance due to their unreliability and general inoperativeness. For example, burners with a bed of unfused particles tend to shift about with any appreciable gaseous pressure so that the gases breakthrough in large streams. This causes large spurts or jets of dame. Further, the materials must be able to withstand normal operating temperatures of around 2GO0 F. and higher.

Another limitation with the conventional porous ceramic block burner is the temperature limitation irnposed by the heat conductance problems back through the ceramic. Excessive heat conductance toward the mixing chamber causes flash-back if the temperature near the mixture chamber becomes slightly too high. Further, if the pressure drops substantially at any time during operation, hash-back will occur in the ceramic block type as is well-known. Not only must .the pressure be maintained above a critical minimum with such burners for flash-back prevention, but conventional ceramic block burners also have a maximum gaseous pressure which is substantially close to the minimum pressure, thereby defining a narrow operating range. Such burners are manufactured either for a high gas pressure application, or for a low gas pressure application, with the operating range being narrow. If low gas pressures occur with if a high gas pressure is imposed upon a low pressure burner, portions of the burner may actually blow-out due to lack of structural strength for the high flow resistance, and also, the flame will readily blow-off (i.e. extinguish) with the high pressure.

it is also of significance that, although the best porous ceramic block burner devised today is somewhat safe from flashback when properly manufactured and when carefully adapted to use within specilic operating pressures, it will readily hash-back when operated outside these special preset conditions.

Another serious limitation with respect to conventional burners is the inability to uniformly heat a large area without complex distribution baies and partitions.'k Even with such baffles, attempts to create a uniform heat over a large surface area have been only slightly successful. Thus, for example, the concept of providing one burner covering `an entire wallarea of a drying room and giving uniform heat over the area is regarded as practically impossible with present burner technology due (l) to the ice tremendous expense of providing proper porous ceramic block of Ithat size, (2) to the uneven heat output from such a burner, (3) tothe complex baiing and multiple gaseous inlets required, `and (4) to other problems inherent with present burners. Even if a burner with a relatively large surface area were devised, the temperature of operation would be relatively low since pressures behind a large surface area would be critical. Further, a slight drop bellow the preset minimum in any portion of the burner could cause dangerous flash-back in the entire mixing chamber area.

Another important factor involving burner technology today invvolves the expanding market in plastics. It is often desired to heat the surface of a plastic sheet or plastic lm coating to soften the plastic .and enable it to be heat sealed, bonded or the like. Present burners are not capable of accomplishing this at a rapid pace in a safe, reliable manner due to temperature limitations, presence of ames (especially uneven flames over the burner surface) sometimes due to the presence of infrared heat,f due to oxidation of the plastic, and other factors. The material obviously cannot safely be passed within fractions of an inch from the burner surface.

Present burners are further not capable of applying intense heat to a local area of an article Without heating adjacent articie portions by radiation, and conductance.

Further, present burners simply are not readily and inexpensively formable to a variety of coniigurations--for example in the shape of a horeshore, or with an unusual pattern on the burner surface, or using three walls of a burner housing as the hot surfaces, or in the form of an undulating burner surface, among others. The difficulty, and often the impossibility, of uniform gaseous distroution over the burner surface or surfaces, also prevents such adaptations of burner structures known today. These limitations prevent liberal use of burners to conform closely to unusual article configurations for soldering, preeld heating, shell mold curing, heat treating, drying and many others, without overheating any local portions of the article.

It is an object of the invention to provide a pressuretype gas burner capapable of providing completely uniform heat over any desired surface, and only in the surface, whether it be a fraction of a square inch or several square feet in area. The surface may further by provided with patterned hot areas and intermediate cool areas if desired. The burner does not need anyV baiiing.

It is another object of -this invention to provide a burner providing uniform heat over an entire area of any desired configuration, even if peculiar. The surface may be curved in two or three dimensions, may be undulating, may comprise one, two, three or more walls of a burner housing, may be hemispherical in nature, or any other desired configuration. The burner emits uniform heat over all the surface area of Whatever configuration without bathing and without projecting flames.

It is still another object of this invent-ion to provide a burner capable of unusually large ranges of temperature and heat output since operable under unusually large pressure ranges. The structur is capable of operating at very high pressures .and flow rates without blow-off, or struc- 'tural failure. It is also capable of operating at a low pressure without hash-back. In fact it has been found that after repeated experiments and weeks of rugged testing of every sort with the novel structure, it has been im- 'to about 12 or so pounds per cubic foot.

possible to cause a flash-back with the burner. burner is known that possesses this remarkable characteristic. The burner simply extinguishes when the pressure falls below the low minimum. Other than the high tem- -perature surface, the burner is cool over the remainder of the structure. The back and sides of the structure can even be touched with the hand without discomfort when the hot surface is at a temperature of up to 2000" F. It does not conduct heat more than a small fraction of an inch into the porous element away from the hottest surface due to the unique characteristics thereof.

It is `another object of this invention to provide a novel burner capable of carefully controlled operation in a iiameless fashion, with or without infrared heat as desired, even at temperatures up to 2090o F. Articles may be passed closely to the burner surface without scorching or danger or disliguration. The burner is capable of providing a high. temperature, continuous, protective mantle of flue gases extending from up to about 1 inch from the burner surface, thereby enabling rapid, protected heating of materials.

Another important object of this invention is to provide a burner having the above attributes and which is also remarkably inexpensive and relatively simple to fabricate.

These and other objects will be apparent upon studying the following specification in conjunction with the drawings in which:

tF-IG. 1 is a perspective expoded view of an embodiment of the inventive surface burner apparatus;

FIG. 2 is a partial, sectional, elevational view of portions of the combustion burner in FIG. 1; and

FIG. 3 is a sectional side elevational view of the burner .assembly in FIGS. 1 and 2.

Basically, the inventive pressure burner comprises a housing pressurized gas and air mixture inlet means in the housing, at least one opening in the wall or top of the housing with a peripheral retaining means, and a layer of fibrous refractory insulating material compressed into a felt-type substance, placed in the housing, retained firmly in position, and sealed over the opening, thereby forming a Wall of a pressure chamber therebeneath within said housing. The layer has an integral, selfsupporting nature, formed from millions of short fibers (eg. 1/16 inch to 1/2 inch long) of refractory material of minute diameter to provide myriads of passages having a size in the micron range to provide a surface having myriads of adjacent minute interstices in the micron diameter range, i.e. normally invisible to the naked eye. It preferably is composed of refractory fibers of alumina and silica, e.g. 50% alumina and 50% silica fibers having a diameter of a few microns (eg. 31/2). The fibers are randomly dispersed in the layer and integrated by compression of the bers and adhering them together with a binder of up to approximately 3% by weight. Itis cornpressed (as by rolling) to the desired density, porosity and thickness for the use involved. The density may vary according to the ratio of the materials, -the gas pressure utilized, and thickness. The important criterion is the uniform porosity, and consistency without surface defects, and the minute pore size, rather than the density, but since these factors are for all practical purposes extremely diicult to accurately determine and define, the density is given in relation to the dimensions and materials involved. With this information as a basis, the substitution of other equivalent materials can be readily determined by one skilled in the `art from this data and the description of the principles of operation given. The thickness may be varied from about 1/16 inch to about 2 or more inches. The density of silica-alumina mixtures may be varied eg. about 2 pounds per cubic foot The ratioV of alumnia to silica may be varied and in certain instances only one of these materials need be used. Additional materials of like characteristics could conceivably be No other added to a felt of silica, or of alumina, or both. The felt must be completely sealed over the housing opening, and is preferably substantially coextensive therewith. A pressurized combustible gaseous mixture, c g. a hydrocarbon gas and an oxygen-containing gas such as cornpressed air is used in the burner. These gases may be mixed prior to injection by any conventional means such as a blower, concentric tubes with adjacent jets, etc. Each of the burner surfaces includes an integral layer of self-supporting, felt-type material formed from short refractory fibers, preferably alumina and silica as in Sil-50 ratio. The felt layer includes an outer surface having myriads of adjacent minute interstices in the micron range size, i.e. so small as to be normally invisible to the naked eye. The felt is integrated into a body with a small amount of binder, up to about 3% by weight. The fibers are randomly dispersed. The mixture of fibers with binder is compressed to the proper thickness and density as by rolling.

The binder may betof different types, c g. ceramic or resinous. The specific binder substance is not critical as long as it does not flow under heat to plug the minute interstices. The fact that a resinous binder, e.=g. a phenolic resin such as phenol formaldehyde tends to break down at Lrfid-600 F. does not detract from its usefulness since the felt is subjected to very high temperatures (up to 2000F.) at only its very outer surface due to its excellent insulating qualities.

The alumina and silica may be fiberized from the molten material by a steam blast in a conventional manner to produce a Iiber which is approximately 31/2 microns in diameter about 1/s to 1/2 inch in length. The diameter or length may vary providing the resulting felt is imparted with the stable body and gaseous ow characteristics explained hereinafter. As stated, density of the 50-50 alumnia-silica felt may vary depending upon the .thickness used, the gaseous pressure, the ratio of substances, and the like. Thus, this mixture can range from about two pounds per cubic foot to about twelve or more pounds per cubic foot for different applications. Preferably the resulting felt body is approximately 1/2 vinch thick but this also may be varied, eg. from 1/16 to 2 or more inches or so, depending upon the factors involved such as pressure desired, temperature of operation desired, porosity and density of the material, area of the burner to` be covered, and rigidity or flexibility of the felt body. A satisfactory material is Cerafelt, sold by Johns-Manville of Manville, New Jersery.

The felt must be tightly and completely sealed over the housing opening. Any seal leaks around the edge createV flame jets which cause the burner to the be undependable for many uses. For example, if it is desired to fuse a plastic coating .on a paper sheet by passing it close to the burner, flame jets would ignite the paper or plastic and ruin the product.

The fel-t is cut to be substantially coextensive with the opening forming the burner surface. It has been found that if the felt is larger than the opening i.e. overlaps the edges of the opening, the felt edges `should be enclosed or else gases tend to be emitted in larger quantities around the edge of the opening due to less flow resistance to the pressurized gases across the-upper corner of the felt. This can thereby, cause a peripheral rim of flame which may project 1A inch or so from the burner surface. This is undesirable for many uses where the article is placed very close to the burner. This feature may not always be objectionable, however.

Within the housing beneath the felt layer is a pressure Vchamber where the pressurized mixture of combustible gases is introduced. The felt material has a substantial resistance to gaseous iiow due to the fact that the gas must pass through myriads of minute tortuous passages having a diameter in the low micron range. Therefore, the pressurized gaseous mixture within the chamber is automatically uniformly distributed over the back of the ii'brous layer without any baffling or partitions being necessary. The gases when passing through the layer do so in a finely dispersed, uniformly distributed manner in the form of myriads of tiny adjacent strearnlets at .a considerable velocity, If the felt layer is not compressed to Cause a high pressure drop, the combustion may occur just .prior to emission of the gases from the outer surface `of the layer to effect an extremely high infrared heat. This infrared heat is eliminated completely by compressling the material to create a large pressure drop across the layer. By so doing, only an extremely thin, blue dame can be seen on the surface and only by looking care- `fully across the burner surface. The pressurized combustion Agases .or iiue gases are emitted at a substantial velocity from the entire surface of the burner and at tempera- -tures of up to 2000" F. or so. Since the entire area supports combustion at each of the minute orifices, the flue gases merge and form .a continuous, air excluding, high temperature mantle of gases. The layer of ilue gas may vary from 1A of an inch or so to about 1 inch or more from the surface ofthe burner.

it is very important in the operation of the novel burner not only that all edges are absolutely sealed, but also that no pin-holes or' a substantial diameter and depth exist in the surface of the felt. Such pin-'holes or surface defes (which are normally visible) cause the Creation of a gas jet with a flame being `ejected a substantial distance from the surface. 'This prevents its proper and eifective utilization and destroys the uniform heating effect. These can normally be visibly spotted before operation. They are clearly detectible during operation due to the creation of flame pimples or spurts. These .pin-'holes must be carefully plugged when they occur or preferably, the felt having pin-holes is replaced by one having none. Thus, it is significant that the felt material be uniform throughout, and that the fibers be lsecurely integrated into the layer to be stable in their position when .subjected to a susbtantial pressure in the pressure chamber. If the pressure in the pressure chamber is only that caused by a mixture of atmospheric air with a gas jet, the pressure drop across the felt will entirely prevent flow of any significant amount, and the vburner will extinguish. This large pressure drop is important to the burner operation. Therefore, the gases must be pre-mixed, i.e. air or oxygen with fuel gas to form an explosive mixture which is forced in a mixed state into the chamber under pressure.

it has een found that the pressure may be varied .over a wide range, relatively speaking, with Athe burner merely responding with a different 'heat output and surface temperature rather than reacting undesirably. For example, the pressure on burners has been varied between a pressure equivalent to 1/2 inch of water per square inch up to l5 `and more inches of water .per square inch without interrupting Ithe operation of the burner. With a 1/2 inch thick layer of 50-50 silica-alumina felt having a density of approximately 3 pounds per cubic foot, pressures below 1/2 inch of water cause the velocity of the ygases passing through the refractory felt to decrease. to the extent where the burner extinguishes. This mm1- mum pressure varies with diiferent size and density layers.

Normally the burner is never operated at such a small pressure, but these figures are given to illustrate the complete lack oi any tendency to hash-back under extreme conditions, and to show the great range of potential operating pressures.. A valve placed on the gaseous inlet containing the pressurized explosive mixture can be throttled down without a second thought, in sharp contrast to .the great danger of throttling a valve on a conventional yburner as is known to everyone in the combustion burner field. Even with such low pressures, no ashback has ever occurred, in fact the inventor has not been able to causev ilash-back, try as he might. During normal operation the pressures would ordinarily be very substantial and considerably above Vthis noted 1/2 inch. water pressure for eliicient operation as a pressure burner, las noted above, but it is of significance to note that even if line pressures on the gaseous mixture should accidentally fall to lthis low value nearv the pressure of an atmospheric type burner, the pressure burner lwill not back-dash like conventional pressure burners. There are probably several cooperative factors preventing hash-back with the novel burner. One reason is due to the excellent insulating quality of the layer due to the minimal conductance through bers a 'few microns in diameter, especially when only a fraction of an inch long, and the inability to lconduct heat between the fibers having a minimal contact. Also, the chemical nature of the refractory, iiberssubstantially reduces their tendency to conduct heat. Thus, the back part of the layer is always coo-l no matter what temperature exists at the outer of front suriface. Further, since the passages .through the felt are tortuous and so minute (ie. low micron range) and the felt is compacted to a state where gaseous flow back through the layer is impossible without a substantial negative pressure differential reverse hot gaseous ilow is never experienced. Moreover the lack of infrared or radiant heat forestalls any possible heat reflectance iuside the pressure chamber.

With the novel type construction, a burner may be formed with a patterned hot surface rather than a rectangular area.

Instead of a flat surface, the novel type burner may ybe formed with curved surfaces.

Not only is the burner unlimited in its possibilities as -to unusual configurations, but it also may be made to `span a large area of many square feet. When manufacturing the burner for commercial sale, certain structure was discovered which enables the novel fburner to be made more quickly, dependably, less expensively, and with a longer life.

One factor which showed up after many days of use was the loss of tiny fibers from the sur-face of the lburner when using coarse screen, eig. about l0 mesh lnconel (0,025 or so openings) as the covering or retention layers. The gaseous pressure tended to blow the fibers out one by one over a long period of time to create a pin-hole type defect in localized areas. rThisv creates visible llame pimples which detract from the burner usefulness. Where line mesh screen was used to retain the 'orous layer, e.g. 40 mesh (0.010 openings) the `fibers did not blow of as before,'bu=t the ine screen when heated, lacked rigidity and tended to bow outwardly away from the iibrous layer a fraction of an inch. As soon as this occurred, portions of the screen bowed into the very hot gaseous areas just off the burner surface Iand glowed red .to thereby seriously detract fro-m lburner quality by creating infrared heat and glowing. Further, when so bowed, it did not return when cooled oi. It no longer served to retain the bers in position.

The structure in FIGS. 1-3 removed both of these weaknesses as well as providing an edge seal to the ibrons layer without the use of adhesives. The assembly lii compirses the components of the main housing i12 with a gaseous inlet for combustible gas and oxidizing gas, a lower coarse mesh support screen lid resting on the peripheral housing shoulder AH6, a fibrous Vceramic layer M3, a compressing, retention, line mesh screen 12d, a rigid, coarse mesh screen 122 coveringrand retaining the line mesh screen andV if desired, a retention ring 124.

The actual mesh of each screen may vary slightly to t ying are even visible.

'iine screen in place to stop any bowing tendency, and

also remove the necessity of wrapping the fine screen inside the housing wall 11d as previously done. The coarse screen is wrapped around the outside of housing 112 to eliminate any tendency for it to create leakage problems around the ed-ge of the felt inside the housing. It may be directly retained as by screws, or may be retained by an outer housing ring 124 held to housing 112 by screws or the like.

The felt layer with its burner, is pre-cut to a width and breadth slightly larger than that of the upper rim of housing 112. Each dimension may be about 1/16 inch larger `on `a burner of about 3 inches square, for example. Thus the layer is laterally compressed in two dimensions as it is placed in the housing. Also the layer is slightly thicker than the corresponding height of the housing between shoulder -116 and screen 129 on top of the housing. For example, a 1A inch layer may be about 1&2 inch thicker, i.e. about so that it is compressed this amount when placed in the housing. It has been found that by compressing the felt in these three dimensions, when the felt lits directly against the walls of the housing without an intermediate screen being therebetween the peripheral area is properly sealed or tightly held to effectively prevent excessive edge leakage under gaseous pressure. Thus even without an adhesive, the burner does not flame up yat the edge but rather the entire surface is at a uniform =heat without a projecting iiame. In operation, the surface burner actually does not even appear to be hot. The front surface is completely black or brown in appearance, no glow is visible, and except for air disturbance visible in the light, no conventional signs of burn- The hand can be placed with cornplete confidence on the back of the burner since it is only slightly above room temperature, while in front the burner surface, the temperature can be up to or well over 2000 F., as tested by a Temple stick, for example. This high temperature zone is invisible and extends to an inch for more away from the f-ront surface, depending on gaseous pressure, felt thickness and felt density. A curious fact is that by-standers have to be warned that the area. in front of the burner is extremely hot and not to be approached since one is tempted to place a hand adjacent the front since it appears completely cool. As with the other forms of the burner, the inventor has actually been unable to create a flash-back, even when purposely trying to do so for test purposes. This factor alone is absolutely unique in the surface combustion burner iield, not to mention its many other advantages. -Since the danger of flash-back is eliminated a throttling control valve 130 may be inserted vdirectly in the conduit f32 which conducts the explosive gaseous mixture from the mixing manifold 134 to the pressure chamber 115 inside the burner housing. This valve can be controlled with complete ease and without worry that the entire apparratus will explode as previously would have occurred if someone had the audacity to install a valve at this location rather than only on the individual air line 140 and gas vline 142 leasing into the manifold.

lother purposes for which gas burners have not been adaptable heretofore.

No catalysts are needed or used in the fibrous layer, nor is any auxiliary igniter system employed to maintain constant combustion, because the pressurized gases, once initially ignited, achieve self-propagating continuous com- ,bustion over the surface of the burner without auxiliary means. It is possible that certain obvious modifications may occur to those in the art upon studying the above speciiication and the forms of the invention illustrated. Such modiiications within the principles taught are deemed to be part of his invention which is to be limited only by the scope of the appended claims and the reasonably equivalent structures thereto.

I claim:

l. A non-glowing pressure-type high temperature combustion burner comprising: a housing; Iconduit inlet means into said housing adapted to introduce pressurized combustible gaseous mixture; an opening means in the wall of said housing; a relatively coarse metal screen mesh in said housing covering said opening and over- `lapping the peripheral edge thereof; an integral felt layer of compressible, fibrous, refractory insulating material in said housing substantially coextensive with said opening means, said material being free of any catalyst; said layer having a uniform porosity and a surface of myriads of adjacent minute orifices free from any pinholes; the iibers of said layer being compacted to an interengaged and integrated state capable of withstanding pressures equivalent to several inches of water per square inch without shifting of said fibers; said felt layer having uniformity of porosity completely therethrough to its outer surface; a sheet of relatively fine mesh screen coextensive with and over said felt layer to retain the inindividual iibers on the surface of said layer from projecting up and blowing off; a relatively coarse mesh, rigid metal screen wrapped over said iine mesh screen and fastened aro-und portions of the outside of said housto hold said iine mesh screen in place and prevent lit from blowing outwardly under operational heat; and said felt layer fitting into engagement on its periphery against said housing, and being initially slightly larger in diameter than said opening in said housing, to seal aganst said housing and prevent edge leakage.

2. A non-glow pressuredype combustion burner cornprising: a housing; conduit inlet means in said housing adapted to provide a pressurized combustible gaseous mixture; an opening means in the wall of said housing; a mesh in said housing covering said opening and overlapping the peripheral edge thereof; an integral felt layer of fibrous, refractory jpsulating material in said housing substantiaily coextensive with said opening means, said material being free of any catalyst; said layer having a uniform porosity and a surface of myriads of adjacent minute orifices free from any pin-holes; the fibers of said layer being compacted to an interengaged and intregated state capable of withstanding pressures equivalent to several inches of water per square inch wthout shifting of said fibers; a layer of relatively fine mesh screen coextensive with and over said layer to retain the individual fibers on the surface of said layer from blowing off; a relatively coarse mesh, rigid screen Vwrapped over said fine mesh screen and fastened around 4the periphery of said inner member; conduit inlet means into said chamber adapted to provide a pressurized cornbustible gaseous mixture thereto; said inner member having a peripheral edge encompassing an outlet opening in the wall of said housing assembly; a supporting screen mesh in said `housing covering said opening; an integral compressible 'felt layer of fibrous, refractory insulating Vmaterial over `said open-ing substantially coextensive therewith; said layer having a uniform porosity throughout, being free of catalytic agents, and having a surface of myriads of adjacent minute oritices free from any pin-holes; the tibers of said `layer being compacted -to an interengaged and integrated state capable of withstanding pressures equivalent to several inches of water 9 if@ pre `square inch Without shifting of said fibers; a layer of Reerenees Citedby the Examiner relatively ne mesh screen coextensive with and over UNHED STATES PATENTS said layer to retain the individual bers on the surface n of said 'layer from projecting from said `surfece `and from 1,3@7651 7/20 Herck 1555**96 blowing oi; a relatively coarse mesh, rigid screen 5 21232:157 8/41 Befgm 117-125 X Wrapped over said line mesh screen and fastened around 3,057,400 19/62 Wagner 158-99 the outside of said inner member to hold layer and said FOREGN PATENTS ne mesh screen in place and prevent it `from bowing outwardly under operational heat; and said outer mem- Aber' being rernovably `secured 'to sald lnner member I@ IAMES W WESTHAVER Pill-mary Examiner' agamst the penpheral edge of said coarse screen to secure E the entire burner assembly. MEYE? PERN, Exmmeh 1,228,433 3/60 France, 

1. A NON-GLOWING PRESSURE-TYPE HIGH TEMPERATURE COMBUSTION BURNER COMPRISING: A HOUSING; CONDUIT INLET MEANS INTO SAID HOUSING ADAPTED TO INTRODUCE PRESSURIZED COMBUSTIBLE GASEOUS MIXTURE; AN OPENING MEANS IN THE WALL OF SAID HOUSING; A RELATIVELY COARSE METAL SCREEN MESH IN SAID HOUSING COVERING SAID OPENING AND OVERLAPPING THE PERIPHERAL EDGE THEREOF; AN INTEGRAL FELT LAYER OF COMPRESSIBLE, FIBROUS, REFRACTORY INSULATING MATERIAL IN SAID HOUSING SUBSTANTIALLY COEXTENSIVE WITH SAID OPENING MEANS, SAID MATERIAL BEING FREE OF ANY CATALYST; SAID LAYER HAVING A UNIFORM POROSITY AND A SURFACE OF MYRIADS OF ADJACENT MINUTE ORIFICES FREE FROM ANY PINHOLES; THE FIBERS OF SAID LAYER BEING COMPACTED TO AN INTERENGAGED AND INTEGRATED STATE CAPABLE OF WITHSTANDING PRESSURES EQUIVALENT TO SEVERAL INCHES OF WATER PER SQUARE INCH WITHOUT SHIFTING OF SAID FIBERS; SAID FELT LAYER HAVING UNIFORMITY OF POROSITY COMPLETELY THERETHROUGH TO ITS OUTER SURFACE; A SHEET OF RELATIVELY FINE MESH SCREEN COEXTENSIVE WITH AND OVER SAID FELT LAYER TO RETAIN THE ININDIVIDUAL FIBERS ON THE SURFACE OF SAID LAYER FROM PROJECTING UP AND BLOWING OFF; A RELATIVELY COARSE MESH, RIGID METAL SCREEN WRAPPED OVER SAID FINE MESH SCREEN AND FASTENED AROUND PORTIONS OF THE OUTSIDE OF SAID HOUSING TO HOLD SAID FINE MESH SCREEN IN PLACE AND PREVENT IT FROM BLOWING OUTWARDLY UNDER OPERATIONAL HEAT; AND SAID FELT LAYER FITTING INTO ENGAGEMENT ON ITS PERIPHERY AGAINST SAID HOUSING, AND BEING INITIALLY SLIGHTLY LARGER IN DIAMETER THAN SAID OPENING IN SAID HOUSING, TO SEAL AGAINST SAID HOUSING AND PREVENT EDGE LEAKAGE. 